The invention pertains to the general field of printing and in particular to inkjet printing.
While there is a considerable variation in the products on offer and the specific technology employed, inkjet printing typically involves expelling small droplets of ink-bearing liquid from miniature nozzles onto the surface of a substrate. Each droplet represents a pixel to be printed. An array of such nozzles is then scanned across (i.e. moved relative to) the substrate in order to address each pixel position. An electronic control unit controls the scanning process and, depending on the image data, sends instructions to individual nozzles as to whether they should print at a given position or time. Because the electronic control unit directs nozzles to expel ink droplets or to refrain from expelling ink droplets based on image data, the ink droplets are said to be xe2x80x9cimage-wisexe2x80x9d expelled onto the substrate. Some color printers use inkjet technology.
FIG. 1 depicts a prior art inkjet head 10 printing on a substrate 12. Inkjet head 10 comprises an array 16 of inkjet nozzles 14. For the sake of clarity, inkjet head 10 is depicted in FIG. 1 as comprising a single one-dimensional array 16 of nozzles 14. The image-wise expulsion of ink from each individual nozzle 14 is controlled by a controller (not shown). The controller moves inkjet head 10 in a scan direction 18 relative to substrate 12 and, using image data, directs individual nozzles 14 to eject fluid ink droplets 20. Repeated emission of fluid ink droplets 20 creates tracks or channels 22 of image-wise printed dots 24 on the surface of substrate 12. Ideally, as exemplified by nozzle 14A, fluid ink droplets 20A are ejected substantially straight from the tips of nozzle 14 to form substantially straight channels 22 on substrate 12.
A problem with inkjet printing is illustrated by nozzle 14E. As shown in FIG. 1, the fluid ink droplets 20E emitted by nozzle 14E exhibit inconsistent trajectories resulting in image-wise printed dots 24E that are not properly aligned in their channel 22E. Inconsistent or off-center expulsion of fluid ink droplets 20 by nozzles 14 may result in printed images that exhibit banding or striations. Inconsistent or off-center expulsion may be may be caused, inter alia, by partially failed or clogged nozzles 14, by aerodynamic forces that change the paths of fluid ink droplets 20, and by xe2x80x9ccross-talk effectsxe2x80x9d between adjacent or closely proximate nozzles 14.
In effort to reduce the inconsistency of fluid droplet emission trajectories, U.S. Pat. No. 4,054,882 (Ruscitto), U.S. Pat. No. 4,219,822 (Paranjpe) and U.S. Pat. No. 4,525,721 (Crean) disclose the use of electrostatic fields to guide fluid ink droplets after they have been emitted from inkjet nozzles.
PCT Application No. PCT/IL96/00150 and U.S. Pat. No. 6,354,701 (the xe2x80x9cKorem Patentsxe2x80x9d) disclose apparatus for ink jet printing involving a printing member patterned with an ink receptive portion having a number of ink receptive dots in a desired resolution and an ink repelling portion that includes the remaining area of the printing member. Fluid ink droplets are image-wise expelled from nozzles onto the ink receptive dots and then transferred from the printing member to a printing substrate.
Intermediate transfer surfaces, such as the printing member of the Korem Patents, have a tendency to retain ink, thereby decreasing ink utilization efficiency, reducing the amount of ink transferred to the substrate and making the intermediate transfer surfaces difficult to clean.
There is a need for inkjet printing apparatus and methods that ameliorate at least some of the disadvantages mentioned above.
In accordance with the present invention, a method for the image-wise transfer of fluid droplets from at least one fluid droplet source onto a substrate is disclosed. The fluid droplets may be water-based or oil-based. If the fluid droplets are water-based, the method comprises ejecting the fluid droplets from fluid droplet source onto a hydrophobic transfer surface which comprises a spatially periodic plurality hydrophobic regions that are less hydrophobic than a remainder of the transfer surface. If the fluid droplets are oil-based, the method comprises ejecting the fluid droplets from fluid droplet source onto a oleophobic transfer surface which comprises a spatially periodic plurality oleophobic regions that are less oleophobic than a remainder of the transfer surface. The method also comprises transferring the fluid droplets from the transfer surface to the substrate by bringing the fluid droplets on the transfer surface into contact with the substrate.
The method may also involve adjusting a spatial registration of the fluid droplets on the transfer surface, wherein adjusting the spatial registration of the fluid droplets on the transfer surface may comprise permitting the fluid droplets to interact with the hydrophobic (oleophobic) transfer surface and at least one of the plurality of less hydrophobic (oleophobic) regions.
The fluid droplet source may comprise a plurality of fluid droplet sources spaced apart from one another by a separation and there may be an integer relationship between a period of the less hydrophobic (oleophobic) regions and the separation of the fluid droplet sources.
The method may involve modifying one or more rheological characteristics of the fluid droplets while the fluid droplets are on the transfer surface. Such modifications may involve: curing the fluid droplets, partially curing the fluid droplets, increasing a viscosity of the fluid droplets, changing a solubility of the fluid droplets, changing a surface energy of the fluid droplets and/or evaporating a solvent contained in the fluid droplets. Such modifications may be accomplished by: irradiating the fluid droplets with electromagnetic energy; subjecting the fluid droplets to vacuum treatment, subjecting the fluid droplets to gaseous flow treatment, subjecting the fluid droplets to chemical treatment and heating the fluid droplets.
The method may comprise modifying sizes of the fluid droplets while the fluid droplets are on the transfer surface.
The fluid droplet source may comprise an ink jet printer head. The transfer surface may be disposed on a cylindrical surface of a drum roller or, alternatively, may be the surface of a drum roller. Bringing the fluid droplets on the transfer surface into contact with the substrate may comprise rolling the substrate against the drum roller.
The transfer surface may comprise a belt member and the method may involve circulating the belt member while ejecting fluid droplets onto the transfer surface.
The less hydrophobic (oleophobic) regions may be periodic in one dimension. They may also be periodic in two dimensions. The less hydrophobic (oleophobic) regions may comprise depressions in the hydrophobic (oleophobic) transfer surface.
Ejecting fluid droplets from the one or more fluid sources onto a hydrophobic (olephobic) transfer surface may comprise making multiple passes between the inkjet head and the transfer surface and, in each such pass, depositing a plurality of fluid droplets onto the transfer surface. The plurality of fluid droplets deposited on each pass may comprise fluid droplets of a different color. The pluralities of fluid droplets deposited during successive passes may be spatially interleaved with one another.
Transferring the fluid droplets from the transfer surface to the substrate may comprise making multiple passes between the transfer surface and the substrate and, in each such pass, transferring a plurality of fluid droplets onto the substrate. The plurality of fluid droplets transferred on each pass may comprise fluid droplets of a different color. The pluralities of fluid droplets deposited during successive passes may be spatially interleaved with one another.
The method may comprise curing the fluid droplets on the substrate, which may involve: irradiating the fluid droplets with electromagnetic energy; subjecting the fluid droplets to vacuum treatment, subjecting the fluid droplets to gaseous flow treatment, subjecting the fluid droplets to chemical treatment and heating the fluid droplets.
Ejecting the fluid droplets from the at least one fluid droplet source onto a hydrophobic transfer surface may comprise ejecting fluid droplets of different colors onto the hydrophobic (olephobic) transfer surface. Transferring the fluid droplets from the transfer surface to the substrate may comprise simultaneously transferring fluid droplets of different colors.
Further aspects of the invention and features of specific embodiments of the invention are described below.